With the divorce rate being as high as it is, many families are getting divorced, finding new partners, then blending families together under one roof.
Many people have unrealistic expectations regarding how smoothly the process of blending will be. Blended families are very different from nuclear families, with different rules, and it generally takes years for a blended family to feel like a nuclear family. For example, in a nuclear family, both parents generally make and enforce family rules. In a blended family, although both adults may create the rules, the biological parent generally enforces them. And a good tip I learned years ago is don’t try to parent a non-biological child until that child has given you permission to do so.
If you want to make the process of blending families go as smooth as possible, consider counseling or even taking a blended-family course.